Cellular Mobile Telephone Systems Generally
A cellular mobile telephone system, such as GSM (Global System for Mobile Communication), essentially comprises a set of basestations and a number of mobile stations or units. (Each basestation is a combined transmitter and receiver station, for 2-way communication with the mobile units.) The basestations are at fixed geographical positions, and the mobile units are generally portable units which can move relatively freely though the region covered by the basestations.
The basestations will of course be coupled together. Typically they are termed Base Transmitter Stations (BTSs), which are grouped into groups each of which is supported by a Base Subsystem Controller (BSC). The BTSs and BSCs together for a Base Subsystem Station (BSS), and the BSSs are in turn grouped into groups each of which is supported by a (fixed) Mobile Switching Centre (MSC). There are fixed communication links (e.g. private lines or the public switched telephone network, PSTN) between the BTSs, BSSs, and MSCs, and between the system generally and other communication systems (e.g. the public telephone system), while communication between the BTSs and the mobile systems is of course by radio. We will term the basestations (or just "stations") plus the associated control stations and links the "fixed system"; the complete system consists of the fixed system plus the mobiles (mobile units).
Obviously the system must support communication with many mobile units simultaneously. Broadly speaking, each station can communicate with mobile units within a region (called a cell) around it. The size of the cell is determined by a variety of factors, such as the transmitter power and receiver sensitivity of the basestation, geographical features such as hills or tall buildings, interference from other basestations, and so on. The cells should overlap to some extent, so that there are substantially no places which are not within at least one cell.
A given system has a frequency band assigned to it, and the frequency band is divided into frequency channels. In an analogue system, a station can communicate with several mobile units at a time using a different channel for each mobile unit. In a digital system, each frequency channel is normally divided into several time slots each of which can carry a single call. The frequency channels in a digital system are sometimes simply be termed "frequencies", with the time slots within those frequencies being termed "channels"; here, we will use the term "channel" to mean a frequency in an analogue system and a frequency, possibly plus a time slot, for a digital system.
A mechanism is required for dedicating a channel to an mobile unit when communication with that mobile unit is required. This is achieved by using a particular channel as a control channel. When a mobile unit is in the quiescent or idle state, with no call being made, it monitors the control channel. If a call is made, the mobile unit and the station initially communicate with each other over the control channel. The station dedicates one of the other channels to the call and passes that dedication to the mobile unit over the control channel. The call itself (i.e. the voice or data communication) then proceeds between the station and the mobile unit using the dedicated channel (with the mobile unit in the active state). The amount of communication over the control channel is small, and this channel can therefore be shared between a large number of mobile units without serious difficulty.
Knowledge of Mobile Locations
In any practical system, it is necessary for the location of each mobile unit to be known, at least approximately, by both that mobile unit and the fixed system. The control channel is used to maintain this system knowledge. Each basestation broadcasts identification information on its control channel, and each mobile unit monitors the control channel. (The mobile units do this on a periodic basis, to minimise power consumption in the idle state.)
Each mobile unit monitors the control channel to identify a cell giving control signals of adequate strength. (A mobile may of course be at a location where 2 or more cells overlap; in that situation, it picks one of the possible cells.) If the mobile unit finds that the control signals from another cell are substantially stronger than those from the cell it is in, it changes to that new cell. Each mobile unit therefore knows where it is in the system, i.e. which cell it is in.
A mobile unit can of course be disconnected from the system, e.g. as a result of being switched off or in a location where it is shielded from or outside the geographical range of the fixed system. When such a mobile enters the system, i.e. first detects signals on the control channel and discovers which cell it is in, it sends a signal to the fixed system announcing its presence in that cell. If the mobile unit then moves sufficiently far through the system, it again sends a signal to the fixed system (via the local station) to update the fixed system on its new position. The fixed system therefore knows where the mobile is.
In practice, it may be convenient to group the cells into groups in the fixed system, with a mobile unit informing the fixed system of a change of location only if it moves from one group of cells to another. (This grouping may match the grouping of BTSs into groups by BSC, or of BSSs into groups by MSC.) Thus the mobile unit will know which cell it is in, but the fixed system will only know the location of the mobile unit to within an area of several cells. So when the mobile unit originates a call, it communicates with a particular station, but if the mobile unit is called, the fixed system may have to cause several stations in turn to try to communicate with the mobile unit.
A mobile may of course move from one cell to another while it is in the active state, i.e. while a call is in progress. The system (either the basestation or the mobile) will recognise this by finding that the signal strength or quality between the mobile and the station falls below an acceptable level. The system is normally designed to enable the mobile to select a new cell and to switch its link from the current cell to the new cell by suitable communication over the control channel, while maintaining the call in progress. In this case, the fixed system will of course be aware of the change of cell by the mobile even if that change is within a single cell group.
Frequency Assignments
As noted above, any particular system uses a particular frequency band, and that band is divided into a large number of distinct frequency channels. These frequency channels have to be distributed among the various stations of the system.
It is of course necessary to keep uplink communication from mobile units to stations and downlink communication from stations to mobile units separate. In some types of digital system, this is achieved by using different time slots in a single frequency channel for uplink and downlink communications. In analogue systems and some other types of digital systems, the frequency band is divided into two separate sub-bands for uplink and downlink communications. However, the two sub-bands generally have identical numbers of frequency channels, and the distributions of the frequency channels of the two sub-bands among the basestations are generally identical. Thus we can generally discuss matters in terms of a single frequency band (i.e. a single set of frequency channels) even if that band is in fact divided into separate uplink and downlink sub-bands.
To maximise the number of frequency channels in a frequency band, the frequency channels are usually packed closely enough together for there to be a danger of interference between adjacent frequency channels. It is therefore necessary to distribute the frequency channels among the stations in such a way that no station has two adjacent frequency channels.
As noted above, each station can communicate with mobile units within its cell; that is how a cell is defined. Also, the cells of the stations of the system overlap, so that there are virtually no places which are not within at least one cell. That means that there will be overlaps between cells, and some places will be covered by at least 3 cells. It is clearly important to minimise the possibilities of interference between adjacent cells, and the distribution of frequency channels among the stations should be chosen accordingly.
A reasonable way of doing this is to divide the frequency channels in cyclic sequence into a suitable number of groups; thus with 12 groups, group 0 will have frequency channels 0, 12, 24, &c, group 1 will have frequency channels 1, 13, 25, &c, and so on. The groups can then be assigned to the stations so that no two adjacent cells have the same group.
The simplest and, in a sense, "ideal" pattern of cells would be hexagonal, which would only require 3 groups to avoid conflict between adjacent cells. In practice, however, cell sizes may differ substantially. For example, it is desirable to match the cell size broadly to the expected density of mobile units (e.g. by suitable choice of the transmitter power of the station). So a large cell may be surrounded by more than 6 other cells, and the number of groups therefore has to be substantially larger than 3.
The cell size and shape will also be affected by geographical factors such as hills, restrictions on where the stations can be located, clutter caused by high density of buildings, &c. There may also be small cells required to achieve coverage of shielded regions within large cells. Some of these factors may affect different frequency channels differently, so the cell size may vary slightly for different frequency channels. Further, the system may develop over time, typically with the addition of further basestations to cope with increased traffic.
A single station location may have 2 or more aerial arrays having different coverages. Logically, i.e. functionally, such a location can be regarded as (and will be treated here) as 2 or more distinct stations, one for each array, each with its own distinct cell.
The assignment of the frequency channels to the stations is normally performed by design engineers, using their skill and experience to estimate the sizes of the cells and the achieve a good distribution of groups among the cells. However, the assignment of frequency channels to the stations is often far from ideal. Thus the actual cell sizes will normally differ to some extent from the estimated sizes, and may change as a result of, for example, building development. The cell size for a particular frequency channel around a particular station may in fact differ considerably from the estimated or average size of that cell. Also, the system may well change over time, with changes in the characteristics of the stations and possibly the addition of fresh stations to cope with increasing traffic.
These effects will often result in the actual cells differing from the estimated cells. Thus a cell may extend beyond one part of the estimated boundary but fail to reach another part of the estimated boundary, there may be a shielded area or "hole" in it, and there may be an isolated area beyond its main boundary resulting from signal splash or skip.
The effect of this is to reduce the capacity and quality of service of the system. In addition, of course, the procedure of dividing the frequency channels into groups which are then assigned to different stations is a conservative technique designed to minimise the possibilities of interference, and it may well be that a particular frequency channel (or several frequency channels) could in fact be assigned to more stations.
It is therefore desirable to have some method of monitoring or measuring the performance, actual and potential, of the system, in terms of cell size, frequency channel distribution among basestations, interference between frequency channels, &c.
Current Technique
System performance can be measured by means of a test mobile which is transported through the regions of interest. A test mobile is a type of mobile unit specialised for testing purposes, and is generally operated by a skilled engineer. It can, for example, be driven along particular roads or around the boundaries of particular regions. As it travels, it can attempt to communicate with various nearby stations over various frequency channels. Detailed information can thus be gathered of how the transmission characteristics for different stations and different frequency channels vary within the region being traversed. This can then be used to adjust various characteristics of the system (e.g. the distribution of the frequency channels among the various stations) to improve system performance.
This technique has certain drawbacks. The test mobile, being specialised for testing purposes, is relatively expensive. More significantly, the process is normally carried out by trained engineers, involving a high degree of skill, time, and labour, and is therefore very expensive.
The general object of the invention is to provide an improved method of monitoring or measuring the performance of a mobile telephone system.